1. Prepare the pan: Preheat your oven to 180°C (350°F). Grease and line a 20-22 cm (8-9 inch) cake pan with parchment paper.
2. Make the batter: In a large bowl, beat the butter and sugar until light and fluffy. Add the eggs one at a time, beating well after each addition.
3. Mix in the vanilla extract: In a separate bowl, combine the flour and baking powder. Gradually add the dry mixture to the butter mixture, alternating with the milk. Mix until smooth. Divide the batter into two equal portions. In one portion, mix in the cocoa powder, cinnamon, and additional sugar until fully combined.
4. Create the marble effect: Pour a layer of plain batter into the prepared pan, then add a layer of the cinnamon batter. Repeat the layers until all the batter is used. Using a knife or skewer, gently swirl through the batter to create a marble effect.
5. Bake: Bake in the preheated oven for 45-50 minutes, or until a toothpick inserted into the center comes out clean. Let the cake cool in the pan for 10 minutes, then transfer it to a wire rack to cool completely.

Dust with powdered sugar or drizzle with icing, if desired. Perfect with a cup of tea or coffee!

Enjoy!